An FM-CW radar has long been used as a radar that calculates a distance and a relative speed between a vehicle and a vehicle in front. The radar emits a frequency-modulated (FM-modulated) signal wave to a target object, detects a mixed wave (beat signal) of a reflection wave from the target object and a transmission wave, and extracts a delay time and a Doppler shift, thereby calculating a distance and a relative speed between the target object and the vehicle.
FIG. 14 is a schematic configuration diagram of a general transmission/reception module and a modulation-signal generating circuit mounted on the above FM-CW radar. The modulation-signal generating circuit includes a voltage control oscillator (VCO) 41 that changes an oscillation frequency according to a control voltage, an FM-modulation-voltage generating unit 42 that inputs a control voltage to the VCO, a modulation-signal generating circuit 40 including the VCO 41 and the FM-modulation-voltage generating unit 42, a transmitting unit 5 that transmits a signal output from the VCO, branches a part of the transmission signal, and outputs the transmission signal, and a receiving unit 6 that extracts a mixed wave of a reflection signal from the target and a divided signal.
The measurement precision when the above transmission/reception module is applied to a distance/speed measuring radar depends on linearity of an FM-modulated signal emitted from the modulation-signal generating circuit 40, that is, modulation linearity of the oscillation signal of the VCO 41. However, it has been difficult, from the cost and technical viewpoint, to obtain high linearity of VCO on the FM-modulation voltage-frequency characteristic (VT-f characteristic).
This kind of VCO requires a predetermined frequency modulation width on a radar system. Therefore, a VCO is structured to have a low setting of a Q value of a tuning circuit, in many cases, and a temperature variation of an semiconductor element at the oscillation circuit side gives a relatively large influence. As a result, a temperature drift of the output frequency of the VCO becomes large, and the VT-f characteristic changes to a frequency axis direction at an ambient temperature.
FIG. 15-1 depicts a general VT-f characteristic, where A denotes a characteristic at a normal temperature, B denotes a characteristic at a high temperature, and C denotes a characteristic at a low temperature. At the normal temperature, the VCO 41 oscillates at a center voltage VA of an FM modulation voltage VT, at an amplitude of ΔVA (operation point PA), and outputs an FM-modulated wave of a frequency modulation width ΔfA. When the ambient temperature increases or decreases, the above operation point shifts to PB and PC, and an FM-modulated wave (range of ΔfB and ΔfC) output from the VCO 41 exceeds a legal frequency range of the Radio Law.
Therefore, conventionally, as shown in FIG. 15-2, the operation point is shifted to a horizontal direction at the ambient temperature, and operation is performed at operation points PA, P′B, and P′C, at each of the normal temperature, the high temperature, and the low temperature, thereby avoiding the above problems. Patent Document 1 mentioned below discloses a conventional technique of constituting the above conventional FM-CW radar device by a modulation voltage circuit that adds the FM modulation voltage, by independently generating a modulation voltage (AC component) and a DC offset voltage controlled at the temperature of the module.
The gradients (modulation sensitivities) of the VT-f characteristics A, B, and C of the VCO 41 at the above operation points PA, P′B, and P′C are different. Therefore, when the same FM modulation voltage (the AC component) is input, the frequency modulation width Δf changes at the ambient temperature, and the beat frequency of the radar device changes. Consequently, a relative distance R from and a relative speed v of the target object cannot be measured accurately.
As a method of solving the above problems, a method of compensating for nonlinearity of the voltage-frequency characteristic (VT-f characteristic) at the control voltage is used. That is, a method of applying an inverse draft of the above VT-f characteristic to the FM modulation voltage output from the FM-modulation-voltage generating unit 42 is used.
FIG. 16 depicts a relationship between a modulation voltage-modulation frequency characteristic (curve F) of the VOC 41 and a time-modulation voltage (correction voltage, curve G) that linearizes the curve F. As an FM modulation linearization technique of the VCO, the Patent Document 2 mentioned below discloses a modulation-signal generating circuit that stores voltage data for correcting the VT-f characteristic in a memory in advance, and reads this data as digital data in a constant cycle, thereby obtaining an analog signal output via a D/A converter and an integration circuit.
Patent Document 1: Japanese Patent Application Laid-open No. H8-146125 (see paragraphs 9 to 18, FIG. 1 and FIG. 4)
Patent Document 2: Japanese Patent Application Laid-open No. 2002-62355 (see FIG. 2 and FIG. 7)